The present invention relates to a support structure for a self-standing storage tank used in a liquefied gas carrier ship, and more specifically, to a technology which avoids problems due to relative movements of a self-standing storage tank and a ship's hull of the liquefied gas carrier ship in the forward and rearward directions, and which improves support of the self-standing storage tank during the movement thereof.
Techniques relating to liquefied gas carrier ships have been disclosed in, for example, Japanese Patent Application, First Publication No. 2-249796, Japanese Patent Application, First Publication No. 4-8999, Japanese Patent Application, First Publication No. 4-92794, and Japanese Patent Application, First Publication No. 4-143187.
An example of these types of liquefied gas carrier ships is shown in FIGS. 10 and 11. This carrier ship has a double casing type (armored-type) ship's hull 1 including a outer shell 1A and an inner shell 1B, and rectangular-shaped storage tanks 3 in holds 2 of the ship's hull 1. The storage tank 3 has a self-standing structure which is independent of the inner shell 1B. By this structure, a carrier ship may be contemplated in which a deck 12 is flattened, the storage capacity is increased, safety during an accident (such as running aground) is improved, and reliability in carrying a stored liquid, such as liquefied natural gas (LNG), is improved.
Each of the storage tanks 3 is placed on a plurality of support blocks 4, as shown in FIG. 11, so that the weight of the tank 3 is distributed. Between the inner shell 1B and the tank 3, fore-aft movement restraining devices for restraining the movement of the tank 3 in the forward and rearward directions, and lateral movement restraining devices for restraining the movement of the tank 3 in the port-starboard directions, are disposed. More specifically, the fore-aft movement restraining devices and the lateral movement restraining devices are disposed between a roof surface 2a of the hold 2 and a roof section 3a of the tank 3, and between a bottom surface 2b of the hold 2 and a bottom section 3b of the tank 3.
FIG. 12 shows the fore-aft movement restraining devices 5 and the lateral movement restraining devices 6, which are disposed between the roof surface 2a of the hold 2 and the roof section 3a of the tank 3. The fore-aft movement restraining devices 5 are aligned in the port-starboard directions (the widthwise direction of the ship), defining the center of a tank dome 9 as a restraining base point of movement. Likewise, the fore-aft movement restraining devices 5 are disposed between the bottom section 3b of the tank 3 and the bottom surface 2b of the hold 2. The fore-aft movement restraining devices 5 restrain the relative movements of the inner shell 1B and the tank 3 in the forward and rearward directions, while permitting the relative movements of the inner shell 1B and the tank 3 in the port-starboard directions.
The lateral movement restraining devices 6 are aligned along the longitudinally extending center line Y of the tank 3, as shown in FIGS. 11 and 12, and restrain the relative lateral movements of the inner shell 1B and the tank 3, permitting the fore-aft relative movements of the inner shell 1B and the tank 3. The lateral movement restraining devices 6 are also provided between the bottom section 3b of the tank 3 and the bottom surface 2b of the hold.
The carrier ship having the support blocks 4, the fore-aft movement restraining devices 5, and the lateral movement restraining devices 6 are designed so as to bear, for example, an acceleration of 0.5 G (gravity) of a collision load. The share of the load is, for example, 0.45 G for friction load at the support blocks 4 disposed at the lower position of the tank 3, 0.02 G at the fore-aft movement restraining devices 5 disposed at the upper position of the same, and 0.03 G at the fore-aft movement restraining devices 5 disposed at the lower position of the same. By the above structure, the safety of the tanks and the carrier ship is ensured, in that external forces caused by waves or a collision and the like are prevented from being imparted to the tank 3.
The structural specifications of the fore-aft movement restraining devices 5 and the lateral movement restraining devices 6 disposed above the tank 3, and those disposed under the tank 3 are different. The reason for this is that the maximum load to be restrained by the movement restraining devices 5 and 6 disposed above the tank 3 is relatively larger than that of those disposed at the lower position, while the required movement maximum load of the movement restraining devices 5 and 6 disposed under the tank 3 can be reduced by friction of the support blocks 4.
FIG. 13 shows an example of the fore-aft movement restraining device 5 disposed above the tank 3. The movement restraining device 5 comprises a pair of stopping blocks 7 arranged on the roof surface 2a of the hold 2 constituted by the inner shell 1B, each block being spaced apart from the other and being formed unitarily with the inner shell 1B, and a chock 8 provided on the roof section 3a of the tank 3 and disposed between the pair of stopping blocks 7. Thus, the relative movements of the chock 8 are restrained by the pair of stopping blocks. In this case, the stopping blocks 7 are welded to the roof surface 2a, and the chock 8 is welded to the roof section 3a. The composition of the lateral movement restraining devices 6 disposed above the tank 3, the fore-aft movement restraining devices 5 and the lateral movement restraining devices 6 disposed under the tank 3 are similar to the above-mentioned movement restraining devices 5.
However, the above-mentioned support structure for a self-standing storage tank may cause the following problems.
That is, as shown in FIG. 14, when the ship's hull 1 bends downwardly in operation, the fore-aft movement restraining devices 5 will work as if they restrain the backward movement of tank 3. Conversely, as shown in FIG. 15, when the ship's hull 1 bends upwardly in operation, the fore-aft movement restraining devices 5 will work as if they restrain the forward movement of tank 3. Because of this, according to the degree of deformation of the ship's hull 1, excessive force will be applied to the fore-aft movement restraining devices 5 disposed at the upper position. Accordingly, there is a problem to be solved in which the movement restraining devices 5 may be easily damaged by loads caused by frequent deformations of the ship's hull 1. Therefore, precise calculations and adjustments are required for the installations of the fore-aft movement restraining devices 5 disposed on the upper position. Furthermore, concerning the chock 8 which constitutes the fore-aft movement restraining device 5 and the lateral movement restraining device 6, particularly in the case in which the chock 8 is welded on an outer surface of the tank 3, substantial analysis of stresses at welded portions is required.